1. Field of the Invention
The invention relates to the field of water craft and, in particular, to a high speed vessel which is supported by a combination of static air pressure, dynamic lift, and aerodynamic ram-effect.
2. Description of the Related Art
The idea of creating a thin layer of air between the water and the surface of a vessel""s hull is not new. The intention is to reduce the friction component of its resistance. The difficulty has proven to be the distribution and control of the airflow. An alternative design is that the vessel also be carried on a pressurized air cushion, at the same time as fiction is reduced.
Many of the presented and patented inventions based on the above ideas lack a description of an overall concept for the invention. Certain inventions related to air cushion vessels demonstrate advantages within limited applications, but at the same time have disadvantages that make them unsuitable for commercial use. For example, they lack a description of how the claimed advantages could be maintained in the sea conditions that could be expected. Far too little consideration has been given to the combination of the requisite comfort, maintenance of speed in waves, reliability and limited maintenance, with high speed and low resistancexe2x80x94requirements that must all be satisfied to produce a commercially successful application.
Usual characteristics of previously presented solutions for air cushion vessels are that;
they focus on methodsxe2x80x94often theoreticalxe2x80x94for improving efficiency by reducing the frictional resistance of the hull when moving in still water
they assume a high level of aerostatic lift
the projected area of the air chamber is often larger than necessary for the displacement (because the size is determined by the arrangement)
the shape in plan view of the air chamber is usually rectangular
they require larger fans to maintain cushion pressure and airflow
the operation is sensitive to sea conditions and the method of sue, their motion is easily affected by passing waves
they have a shallow draught and large air leakage, particularly in waves
they have low hydrodynamic damping at speed
they have comparatively small reserve displacement to be able to counteract changes in trim
the hulls are extremely mechanically complex and are complicated to manufacture, which makes construction expensive
they require complicated control systems, and contain construction elements that lead to considerable maintenance costs and give less reliable operation
On both hovercraft and SES, the air cushions have been contained by flexible enclosures, called xe2x80x9cskirtsxe2x80x9d. These have proven to be susceptible to wear and damage, and required regular replacement, in SES particularly the fore skirt. The small number of vessels built has led to spares being expensive. Using flexible skirts involves continuous air leakage under the skirt, although in controlled amounts. The airflow leads to additional energy consumption and an increase in the total power required for propulsion. Large variations in the leakage cause pressure fluctuations/pressure drop in the supporting air cushion, which can lead to considerable variation in the hull resistance and vibrations in the vessel (reduced comfort). A shallow draught when the vessel is cushion borne increases the likelihood of air leakage in a seaway. On hovercraft and SES, the forward skirt section projects an appreciable and blunt surface in the direction of motion, which, in a head sea and bow wave, can give rise to a relatively large increase in the resistance, i.e. a considerable reduction in speed. Waves that hit the skirt cause pressure variations in the air cushion, which are transmitted to the vessel and reduce the ride comfort.
Several inventions have been put forward in which flexible enclosures of the air cushion are totally or partially replaced by rigid ones, e.g. by Burg, Barsumian, Harley, Bixel, Peters and Stolper, for use on single and multihull vessels. Barsumian, Bixel and Harley combine a limited amount of dynamic lift (depending on speed, deadrise, trim and surface) on a (preferably forward) hull section in contact with the water, with aerostatic lift (air cushion), mainly located in the aft portion of the vessel. When planning, these inventions try to achieve a minimum of hull surface in contact with the water, in order to reduce the frictional resistance. Burg and Bixel extend the air chamber along almost the entire length of the hull at the water line. (Peter""s idea is a conventional SES, in which the invention comprises a moveable, athwartships division of the cushion chamber, which can be activated for motion control.). In Stolper""s idea, the intention is that the wind created by the movement of the vessel will be used for natural ventilation/air lubrication of planning, supporting surfaces, rather than the air cushion technique. The other inventions mentioned require forced ventilation (fans). A common factor of the above vessel solutions is that they have a minimum of hydrodynamic and hydrostatic (reserve displacement) lift and motion damping in relation to that which is needed to create a seaworthy vessel, and that they are primarily designed to reduce the wetted area, i.e. frictional resistance. Several inventions (in particular those of Burg) are considerably complex, demanding increased maintenance to provide reliability.
Vessels supported on air cushions are propulsively more efficient at high speeds than displacing and planning vessels. For devices intended for propulsion by contact with the water, high speeds involve hydrodynamic complications (cavitation, reduced efficiency, erosion damage), which can increase further due to air leakage from the cushion, which in turn is dependent on the vessel""s position in the water/trim at speed. Sporadic ventilation of the propulsive device gives rise to torque and pressure fluctuations, which in turn can cause operating damage to the gears and engines.
Consequently, almost all of the ideas described above require propulsion using surface-piercing propellers (the blades of which have a fully ventilated suction side) as shown in figures in the respective patents. Unless stated otherwise, surface-piercing or air propellers are used. For moderate speeds, also a conventional propeller can provide propulsion.
Experience has shown that surface-piercing propellers have a shorter operating life than conventional propellers. Variation of blade immersion in the water can easily lead to load variations in a seaway, for both propeller and engine, which necessitates a dynamic regulation system. Precise regulation of the blade immersion is also necessary during acceleration and when passing the xe2x80x9chump speed,xe2x80x9d not to overload the engine at low revs. Too high a torque can easily lead to the vessel not being able to reach full speed (xe2x80x9cgets stuck on the humpxe2x80x9d).
Only Burg refers in one of his patents to the possibility of using water jet propulsion, but puts the water inlet in an unsuitable position for its function, and does not develop propulsion alternatives further. The mixture of air into the water usually results in a greater likelihood of cavitation, thrust reduction, propeller ventilation and reduced propulsion efficiency. The surface-piercing propeller is the best choice, as it is designed for these conditions, and thus works at high speeds. However, it has poor characteristics during low speed maneuvering and when reversing, which increases maneuvering time in harbor. This, plus large load variations in the propulsion unit, has limited its commercial application. Ordinary propellers, with wing profile blade sections, can be used up to approximately 40 knots (oblique flow when mounted on an angled shaft usually leads to erosion damage due to cavitation). Higher speeds require a propeller shaft aligned with the direction of flow, modified blade sections or fully cavitating blades with lower efficiency. Contra-rotating propellers can be around 10% more efficient than a single propeller. They can work at speeds above 70 knots, but at these speeds a slender strut and gear house are required, to reduce resistance and cavitation damage.
Air propellers could be a possible alternative for high speeds, where no other method of propulsion can be used. Burg has mentioned propulsion using air propellers in one patent.
Both surface-piercing and contra-rotating propellers have been produced for only relatively low powers, so propulsion systems that operate at high power output and high efficiency at these speeds are of the utmost interest. In practice, complicated propulsion systems have given reduced system efficiency and unreliable operation. Consequently, there is a need to develop new hull solutions, which permit the use of proven and reliable propulsion systems and which, above all, are designed to be part of an efficient total solution.
The commercially accepted and most well proven propulsion system for high-speed vessels is currently water jet (WJ) propulsion. Efficient WJ installations for powers over 20 MW and speeds approaching 80 knots currently exist, and units for up to 50 MW are under development. To date, WJ has been installed in conventional SES, but with some difficulties in avoiding air leakage from the cushion to the WJ unit, which leads to reduced efficiency and cavitation damage to the pump, and damage to the main engines as a result of load variations.
The purpose of the invention is to specify the design of a vessel hull that combines air cushion technology and improved propulsion efficiency with increased comfort, good maneuverability, good stability and safety, in both still water and normal sea and operating conditions, to provide an effective total solution that also satisfies the demands of commercial application.
The idea behind the invention is suitable for application in both monohull and multihull vessels, for both military and civilian use, to an extent that is based on how each vessel is used. The idea as such is not restricted to any absolute vessel size; it is practical considerations like the strength of the building materials, hull weight, available fan capacity, existing propulsion system etc. that limit the application. It is also expected that the relative speed will normally be limited to a group of vessels with service speeds in relation to vessel length that are higher than that expressed by a length Froude number of approx. 0.6. This is comparable with the speed range for such vessels, which are normally referred to as xe2x80x9csemi-planingxe2x80x9d and xe2x80x9cplaningxe2x80x9d.
Experience from previous air cushion vessels shows that, with a smaller cushion area and a moderate air pressure, it is possible to support the same proportion of the vessel""s displacement as a hovercraft or an SES. This, combined with the fact that it is possible to divide the air cushion between two or more hulls, can be used to make each air cushion hull and air cushion chamber slimmer (greater length/width ratio) than for conventional air cushion vessels, so that the hulls have a reduced resistance, are more seakind and seaworthy, and create a safer vessel with greater comfort and less speed reduction in waves. The same conditions can be used to combine the air cushion hull with a more conventional, planing forebody, which has the combined job of confining the air cushion using a rigid construction, generating a limited dynamic lift in addition to the lift of the air cushion, containing reserve displacement for large vessel motion and generating a damping of this motion.
The proposed invention is also expected to suffer less speed reduction in a turn than a conventional SES, because: here is better confinement of the air cushion using rigid side walls, which go below the water surface, around the cushion chamber and the bow and stern planing surfaces counteract changes in the vessel""s trim, which reduces the likelihood of air leakage from the air cushion, which would lead to the vessel sinking lower in the water, increasing the resistance.
A feature that distinguishes this invention from other solutions is that the supporting air cushion hulls will be combined with hull sections called propulsion hulls, which are integrated into the vessel. These are specially intended to house the propulsion units for the vessel. The propulsion hulls will be specificallyxe2x80x94although not exclusivelyxe2x80x94designed to use the most well proven and accepted propulsion system for high speed and high power, i.e. water jet propulsion. Other propulsion systems can also be used with the above hull configuration. The propulsion hulls will also be designed and located with regard to the behavior in waves of the air cushion hulls, although they themselves affect the seagoing characteristics of the vessel. In addition inherent resistance of the propulsion hulls will be taken into consideration, as well as how they alter the pressure interference between the hulls. This could lead to the propulsion hulls being designed with both a typical planing hull shape and dimensions, and with an almost purely displacing hull shape (round bottom) and dimensions, depending on the conditions the concept is to be adapted for.
In comparison with hovercraft and SES, this invention provides greater freedom to balance the lifting capacity of the air cushion, the vessel""s planning and displacing hull elements and the wetted area of the vessel (frictional resistance) against improvements in other characteristics, such as comfort and speed reduction in waves, and also to adapt the vessel for proven and reliable propulsion systems, e.g. water jet propulsion. By the choice of the plan form of the air cushion, in combination with partially adjustable surfaces for generating dynamic forces, it is possible to achieve a better balance between weight distribution and the design of the vessel, in order to improve the seakeeping characteristics and create passive hydrodynamic motion damping, which is normally not present in other air cushion vessels.
In multihull applicationsxe2x80x94depending on the geometryxe2x80x94there is often a reduced air flow velocity, and thus increased static air pressure in the constriction between the hull, wet deck and water surface (ram effect). For small vessels and high speeds, the resulting aerostatic lift can be considerable. In the proposed invention, it is intended that it will be possible to regulate the pressure using a device to restrict the airflow. The device comprises an inflatable elastic bag or bags, connected to each side hull and the wet deck. The bags control the cross section of the air channel, the flow velocity and pressure rise, and thus make it possible to optimize any reduction in the total resistance of the vessel. The aerostatic lift on the vessel, and the resulting reduction in the water resistance, must be weighed against the total increase in the air resistance for the vessel which is also considerable at high speed when the air velocity is reduced.
For high payload and/or low draft operation a second elastic bag system may be fitted to enclose the gas volume between the demihulls in the bow. The resulting center air cushion may be pressurized with a separate fan system to give additional lift/payload capability and/or reduce draft.
The basic configuration of the solution has fewer moving parts, resulting in less maintenance. The systems that are being proposed in developments of the idea are of a type that allows the basic concepts to work even when they are not operating, which increases reliability.
The recommended arrangement makes it easier to find space to install a motion damping system (e.g. a hydrofoil system), if this should prove necessary, than is the case in a hovercraft or SES.
The invention comprises a vessel hull, which is supported by a combination of static air pressure (an air cushion), dynamic lift (planing hull section), aerodynamic ran-effect (in multihull applications) in combination with additional hull sections (some of which are planing and some of which are displacing) designed to improve the stability, forward motion and seakeeping characteristics of the vessel, house efficient propulsion units and reduce the resistance of the vessel, not only in calm water but equally in waves. The idea of the invention can be used for both monohull and a number of multihull vessels described below, intended for high relative speeds.
In comparison with hovercraft and surface effect ships (SES), this invention provides greater freedom to balance the lifting capacity of the air cushion, the vessel""s planing and displacing hull sections and its wetted area (frictional resistance) with improvement of other characteristics, such as comfort and speed reduction in waves, and to adapt the vessel for proven and reliable propulsion systems, such as water jet.
By selecting the plan form of the air cushion in combination with partially adjustable surfaces for generating dynamic forces, it is possible to achieve a better balance between the vessel""s weight distribution and shape, in order to obtain better seakeeping characteristics, and create passive hydrodynamic motion damping, which are normally not present in other air cushion vessels.
Another preferred embodiment is a multihull vessel as described above, constructed for high payload to size ratio and/or low draft operation, where the volume between the side hulls are enclosed with an enclosing arrangement as mentioned above at the aft end of the tunnel, and a similar simi-flexible enclosure arrangement at the bow of the vessel. The resulting volume is pressurized with a separate fan system, of a similar type as outlined above, to give air cushion lift from this arrangement in addition to the air cushion lift from the within the demihull located aircushions. The cushion pressure of the center cushion will normally be approximately 50% of the pressure in the cushions in the demihulls. The arrangement can be shut off if required.
The deck structure of the mentioned vessel is characterized in that the underside of the deck structurexe2x80x94the wet deckxe2x80x94will normally be fitted with a volume body, positioned symmetrically around the vessel""s fore and aft plane of symmetry, with the purpose of reducing water impact on the wet deck and providing reserve buoyancy in the event of diving.
Another preferred embodiment of the vessel hull according to the invention is a vessel hull as the one described above, but in which the propulsion hull is a separate, symmetrical hull, positioned between the single hulls, and in which the single hulls can be symmetrical or asymmetrical.
Another preferred embodiment is a vessel hull in which the propulsion hull is a separate, symmetrical hull, positioned between the single hulls, and in which the single hulls are asymmetrical, and where the width of the propulsion hull is equal to the distance between the inner sides of the single hulls. The bottom of the forward part of the propulsion hull is connected to the forward planing bottom surfaces of the respective single hulls. The propulsion hull is of equal length or longer than the single hulls and extends ahead of the foremost end of the single hulls.
Another preferred embodiment is a catamaran in which propulsion hulls are located inside the air cushion chamber of each single hull and in which the single hulls are symmetrical. These propulsion hulls comprise two planing bottom surfaces, connected to one another in a fore and aft plane of symmetry, two side surfaces, which are connected to the bottom surfaces and which form an acute angle with an arbitrary vertical plane, and a transom surface that is a transverse plane relative to the fore and aft direction of the vessel. The transom will be in a fore and aft position that is in or near the transom of the air cushion hull. The propulsion hull is connected to the roof of the air cushion chamber.
Another preferred embodiment is a catamaran in which propulsion hulls are located inside the air cushion chamber of each single hull, and in which the single hulls are symmetrical.
The invention further comprises a water lock installed in the step at the fore containment surface of the air chamber, in order to limit leakage from the air cushion. This comprises blowing out water at high velocity (impulse), vertically or obliquely aft in a sheet along the boundary line of step along the bottom surface of the vessel. The jet of water forms an angle between 0xc2x0 and 90xc2x0 with a vertical plane, across the fore and aft direction of the single hull, but preferably 60-70xc2x0. This will preferably be used in a seaway, and it should be possible to shut it off.
Another preferred embodiment of the invention comprises an air lock of the same design and function as described above, but in which water is replaced by air.
Another preferred embodiment is a catamaran with a groove between the inner side of each single hull and the vertical side of the corresponding propulsion hull, and in which this groove is intended to collect air that leaks over the inner side of the bilge keel of each single hull.
Another feature of the invention is a vessel fitted with stepped volume bodies in the fore body, positioned on one or both sides of the single hulls above the planing bottom surface, to provide reserve buoyancy and to deflect water from the sides of the hull, and as a result reduce the water pressure on the wet deck.
Another preferred embodiment of the invention is a vessel in which the cushion chamber in each single hull is divided up by, mainly vertical, solid or perforated bulkheads, ruining fore and aft and athwartships, which extend from the ceiling of the cushion chamber down to above the water surface, which forms the lower containment surface of the air cushion, in order to limit the speed of the pressure equalization that takes place when air leaks from the cushion or from one or more bulkhead sections. Each section can have a separate air supply.
Other preferred embodiments are multihull vessels, in which there are more than two single hulls, i.e. three, four, five or more, and in which the propulsion hulls are positioned on the side or sides of each single hull, or as separate propulsion hulls between the single hulls, and/or with propulsion hulls located inside each cushion chamber, or with a combination of side-connected and separate propulsion hulls between the single hulls or inside the cushion chambers, on the same principles described above.
Another preferred embodiment is a multihull vessel as described above in which the cushion pressure in each symmetrically positioned pair of single hulls can be regulated separately, in order to counteract an external heeling moment acting on the vessel.
Another aspect of the invention comprises a vessel with two or more air cushion hulls, in which the fans are connected together by air ducts within each hull and/or between the hulls, so that if one fan breaks down, another fan will be able to compensate for its air supply, so that the air cushion concept continues to operate, albeit to a lesser extent. Normally, with all fans running, all of the connecting ducts will be shut off at both ends by valves located at the outlet of each fan.
Another preferred embodiment comprises a vessel hull comprising just one single hull (monohull), in which the propulsion hull is located inside the cushion chamber of the single hull, and in which the single hull and the propulsion hull are symmetrical.
Another preferred embodiment comprises a vessel comprising just one single hull (monohull) with two propulsion hulls connected to either side the single hull and having grooves, as described above. Another embodiment further comprises in addition a symmetrically positioned propulsion hull inside the air cushion chamber of the single hull.
Other preferred embodiments comprise vessels in which the length of the air chamber in each single hull, from the transom to the step, makes up between 45 and 85% of the length of the hull.
Another aspect of the invention is a multihull vessel with a high propulsion-speed to weight ratio, in which aerostatic lift on the wet deck is used (ram-effect); and which has an arrangement comprising at least one air restricting device an inflatable elastic bag or bags, which is/are connected to either side hull and the wet deck, in order to control the airflow and pressure build up, in order to achieve the balance between lift and air resistance that reduces the total resistance of the vessel. The at least one air restricting device can e.g. comprise an inflatable bag or bags. The inflatable bag car bags can be made of e.g., rubber and/or plastic.